1. Field of the Invention
The present invention relates to a method for forming an electrode for Group-III nitride compound semiconductor light-emitting devices. The term “Group-III nitride compound semiconductor” used herein covers n- and p-type compound semiconductors doped with any element; compound semiconductors containing a Group-III or —V element and at least one selected from the group consisting of B, Ti, P, As, Sb, and Bi; and compound semiconductors represented by the formula AlxGayIn1−x−yN, wherein x, y, and x+y are zero to one.
2. Description of the Related Art
Conventional group-III nitride compound semiconductor light-emitting devices have low light extraction efficiency. The following documents disclose various techniques for forming electrodes from metals having high visible light reflectivity: Japanese Unexamined Patent Application Publication Nos. 2003-086843, 2004-179347, 2005-011857, and 2004-140052.
A conventional Group-III nitride compound semiconductor light-emitting device (light-emitting diode) of a face up-type will now be described. FIG. 8 is a sectional view of the Group-III nitride compound semiconductor light-emitting device, which is represented by reference numeral 900. The Group-III nitride compound semiconductor light-emitting device 900 includes a sapphire substrate 10, a buffer layer (not shown) made of aluminum nitride (AlN), an n-type GaN layer 11 doped with Si, an n-type AlGaN clad layer 12 doped with Si, a light-emitting layer 13 having a GaN/InGaN multi-quantum well structure, a p-type AlGaN clad layer 14 doped with Mg, a p-type GaN layer 15 doped with Mg, and a p+-type GaN layer 16, these layers being deposited on the sapphire substrate 10 by metal-organic chemical vapor deposition (MOCVD) or metal-organic physical vapor deposition (MOPVD) in that order.
The p+-type GaN layer 16 is substantially covered with a translucent electrode 21 made of indium tin oxide (ITO). The translucent electrode 21 is partly covered with a pad electrode 22 made of gold. An n-electrode 90 including a vanadium (V) layer 91 and an aluminum (Al) layer 92 is disposed on the n-type GaN layer 11. Light is extracted through the translucent electrode 21; hence, the Group-III nitride compound semiconductor light-emitting device 900 is a face up-type light-emitting diode (LED).
N-electrodes are usually made of aluminum, which is inexpensive. Aluminum has high reflectivity for near-ultraviolet to visible light and is suitable for electrodes for light-emitting devices. However, the bonding strength between aluminum and gallium nitride or the like is not necessarily high; hence, an aluminum electrode should not be directly formed on a GaN layer but needs to be formed on a contact electrode layer which is made of another metal and which is disposed on the GaN layer.
With reference to FIG. 8, the vanadium layer 91 has a thickness of about 20 nm (200 Å). Investigations performed by the present inventors have shown that the presence of the vanadium layer 91, which has a thickness of about 20 nm (200 Å), allows the reflectivity of the interface between the n-electrode 90 and the n-type GaN layer 11 to be about 40%, that is, the presence of the vanadium layer 91 causes serious light absorption. In usual, an n-electrode occupies about 10% of the area of a horizontal surface of a light-emitting device. This n-electrode occupies such a large area and has an absorptance of about 60%; hence, the influence of this n-electrode on the light extraction efficiency of the light-emitting device is non-negligible.